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Nitrogen: Balancing Aesthetics, Functionality, and Environment

Published in the April 2011 IssuePublished online: Apr 24, 2011Bryan Hopkins

"Bryan Hopkins, soil scientist

Nitrogen is an essential nutrient for all living things – turfgrass being no exception. Leading edge turf managers need to understand more than this fact – becoming better managers if they have a rudimentary understanding of the chemistry, physics, and biology behind this important element.

NITROGEN CYCLE

The atmosphere is composed of 78 percent nitrogen gas, but this gas is inert and most plants can’t use it until it is converted another form. In nature, this is done by legumes (such as peanuts, clover, alfalfa, etc.), certain microbes, and lighting, but the amount that is depositing naturally only amounts to less than a quarter pound of nitrogen per 1,000-square-feet annually. Maximizing growth of most cultivated plants requires 8-20 times the amount provided naturally, with fertilizer needed to make up the difference.

All living things contain 1-5 percent nitrogen, which is recycled upon decomposition. This nitrogen is mostly chemically bound to carbon-containing molecules. Some of these materials, such as woody tissues, are very slow to decompose – taking years to completely release all of the nitrogen and other nutrients. Others, such as a blade of grass, mostly decompose within a few days to weeks.

Urea fertilizer also has nitrogen bound to carbon, but it is released rapidly when moisture and the urease enzyme is present (which is almost always the case when in contact with soil). Coating the urea or mixing with a urease inhibitor can result in the nitrogen being released more slowly.

Nitrogen from carbon-containing compounds converts to ammonia gas, which can be lost to the atmosphere or converted to the ammonium form of nitrogen when the ammonia is found in the soil solution. If the soil “captures” the ammonia and converts it to ammonium, this form of nitrogen quickly converts to the nitrate form within hours to days. Plants can take up both ammonium and nitrate, but not urea or other forms of nitrogen. It doesn’t matter whether the N comes from inorganic fertilizer or steer manure, it all ends up in the same form before plants can utilize it.

LOSING NITROGEN

Nitrogen can be lost to the atmosphere or leached from the rooting zone. Both of these loss mechanisms are environmental concerns. And, both types of loss represent a waste of resources.

As mentioned previously, ammonia gas is formed before it converts to ammonium. In order to avoid losing nitrogen via ammonia volatilization, it is important to water urea fertilizer into the soil (with a minimum of a quarter inch of water) and/or incorporate it into the soil.

Once nitrogen converts to nitrate, it is again susceptible to loss. It can be lost to the atmosphere due to the process of denitrification. Nitrous oxide (laughing gas) is given off to the atmosphere and this is a potent greenhouse gas (thus the environmental concern). The denitrification process is microbe driven and occurs in anaerobic conditions (oxygen depleted). Most soils have at least pockets of oxygen deprivation, but the rate of nitrogen loss to denitrification increases dramatically in water logged soils.

Nitrate can also be lost due to leaching below the rooting zone due to the fact that it has a negative electrical charge, which is repelled by the soil that also has a negative charge. This is in contrast to ammonium, which is positively charged and, therefore, attracted to the soil. The environmental concern for nitrate leaching is that it can reach ground or surface water supplies. Drinking water high in nitrates can be toxic, especially to human infants.

IMPROVING NITROGEN USE EFFICIENCY

The key to reducing nitrogen losses is to insure that plants have a steady, non-excessive supply of this nutrient.

This is particularly challenging in turfgrass applications due to at least three reasons: 1) shallow rooting depth; 2) high rates of water commonly applied, and 3) predominance of sandy soils in many sports turf applications.

Application of a rapid-release nitrogen fertilizer can easily be lost within days/weeks of application, especially on sandy soils, rendering the turf the nitrogen deficient.

Nitrogen fertilizer should be applied when plants are actively growing. The source should be a slow or controlled release formulation that has been proven through independent research.

Alternatively, the nitrogen can be injected into irrigation water to serve the same purpose of steady supply. The other practice that should be employed is good irrigation management to avoid leaching and denitrification losses. Following these practices improve nitrogen-use efficiency by 30-50 percent over the average nitrogen management program.

Traditionally, cool-season turf managers have been told to apply 3-5 pounds of nitrogen per 1,000-square-feet annually, with about half of it applied in the fall and the remaining spread evenly through the spring and summer. Many managers found that they would get spikes of lush, green growth followed by periods of color decline.

In response, they would increase their nitrogen fertilization significantly. Unfortunately, this results in poor root growth and, therefore, turf that is not as resistant to traffic pressure and is not knit together as well. In the short term, the turf might look good with excessive N application. But, the long-term result is poor-looking turf that is not very functional.

Instead, the best approach is to fine tune irrigation management and insure a steady supply of nitrogen with relatively higher rates during the fall when shoot growth slows, but roots are still active.

Managers following this program can get by with 3 pounds of nitrogen annually for most landscapes where minimal mowing is desired and 4-5 pounds of nitrogen for more intensive applications. Sand-based turf will require slightly more nitrogen than heavier textured soils.

Figure one shows the collective results of fertilization and irrigation research trials resulting in better season long color and reduced mowing, as well as reduced nitrogen losses with associated environmental impacts.